An electric power-assisted steering of a motor vehicle has an electric power assistance motor, by which a torque or a force, amplifying the effort manually exerted by the driver of the vehicle on the steering wheel, is applied on the steering column or on the steering pinion, or even directly on the rack. When the assistance function is used, an electric current with a more or less important intensity flows through the windings of the electric power assistance motor, the intensity of this electric current depending on various parameters, according to preset “laws of power assistance”.
In certain driving or operating circumstances of a motor vehicle, the rack hits a mechanic end of travel stop, with a relatively high speed. This results in shock and noise. In fact, the reaching of the mechanic stop causes a very sudden deceleration, the speed of the rack moving from a high value to zero over a very short distance, thus corresponding to a vibratory excitement of the steering system, just like a “hammer blow”.
These undesirable effects are amplified in the case of a power-assisted steering if the laws of power assistance are not adapted to such a situation of arrival at the end of travel.
French Patent FR 2 810 614 already proposes to electronically control the maximum intensity of the electric current flowing through the power assistance motor, based on the value of the rotation angle of the steering wheel or of an equivalent quantity provided by a sensor, so as to reduce or limit this maximum intensity when the steering system reaches the steering lock stop.
More particularly, the French patent 2 810 614 provides that an electronic control device, particularly an electronic computer, reduces the maximum intensity of the electric current flowing through the power assistance motor based on the rotation angle of the steering wheel or an equivalent quantity, according to a programmed curve. This maximum intensity may thus continuously decrease, beyond a predetermined rotation angle of the steering wheel, to a reduced value applied when the steering lock stop is reached.
This known method is particularly intended, to reduce the intensity of the electric current flowing through the power assistance motor, when the end of travel position is maintained, so as to preserve the power assistance motor by avoiding an excessive heating of the latter. However, such a method does not take into account dynamic parameters such as the speed of rotation of the wheel, in order to also reduce the shocks and noise at the end of travel.
The U.S. Pat. No. 6,408,235 patent discloses a similar method, in which not only the rotation angle of the steering wheel, but also the angular speed of the steering wheel can be taken into account, the actual angular speed being compared with a “target” angular speed and the power assistance torque being adapted based on the difference of these speeds, in a closed loop control. Thus, the end of travel shocks can to a certain extent, be reduced.